1. Field of the Invention
This invention relates to a lens meter measuring a focal length of the lens, and is especially adapted to provide digital indications for non-spherical lens having two focal points, such as a lens for astigmation.
2. Description of the Prior Art
Commonly diopter D is employed to indicate refraction of the lenses for glasses by way of the reciprocal of the focal length F, according to the equation D=1000/F. The focal lengths of lenses for glasses are usually indicated in diopter values of plus or minus.
In conventional lens meters, relative positions within an optical system along the optical axis are changed, and by varying the distance of the light beam source to change the focal plane of the corona image, the optician observes the focal position by naked eyes or an eye inspection device to determine the focal length according to the relative relationships in distance between the of the corona images and light emitting system. However, such visual device provides an observational error in the determination of the focal positions due to the individual conditions, and thus accuracy of the measuring is not satisfactory. In such conventional devices, it is particularly difficult to sense and read each focal plane of a lens having two focal points using corona images to determine the exact focal distance of the objective lens to be checked.